1. Field of the Invention
This invention relates to a stiffening element which possesses stiffness and strength with a minimum of weight, such stiffening element being particularly useful in the manufacturing of the contoured curved surface of a boat hull, the stiffening element comprising the intermediate layer between settable resin impregnated glass fiber sheets. More particularly, the present invention is directed to such stiffening element and method of fabricating a boat hull therewith, wherein the stiffening element comprises a layer of plastic foam with top and bottom skins or sheets, preferably of paper, the laminate having a plurality series of slits which provide deformability and flexibility in a plurality of directions.
2. Description of the Prior Art
Laminates of a plastic foam between outer sheets or skins of paper or cardboard have been previously developed, principally as a material for use in the production of cartons or similar containers. This type of construction is illustrated, for example, in U.S. Pat. No. 2,770,406 to Lane, which illustrates a carton formed of a plastic foam sandwiched between two outer sheets of paper, preferably Kraft paper.
This type of sandwich construction has also made its way into the boat industry as illustrated, for example, by U.S. Pat. No. 3,092,851 to Sauer. The boat illustrated in this patent is made from a waterproof board or heavy sheet material which can be readily scored and folded along predetermined curved fold lines, with a particularly suitable sheet material comprising a laminate of paper and polystyrene foam, a preferred material being one sold under the trademark FOME-COR. In accordance with the Sauer patent, the flat foldable board is scored or otherwise processed to define a series of foldably connected panels which form the bottom, sides and top of the boat. It is suggested that, by using a foldable board, one avoids the problem of seams below water level, and, through the proper shaping of the panels, one can avoid or at least minimize the need for framework or bracing.
The usefulness of a boat fabricated from a laminate of plastic foam between outer skins or sheets of paper or board is, of course, quite limited. And, this type of construction is apparently useful only where strength and rigidity are not required. However, due to the buoyancy characteristics of plastic foams, such plastic foams have in the past been made an integral portion of boat construction, particularly where other construction elements are present to provide the desired strength and rigidity characteristics, the plastic foam allowing the production of a boat hull having a high strength to weight ratio. Representative examples of this type of construction are illustrated in U.S. Pat. No. 3,668,051 to Seemann and U.S. Pat. No. 3,687,768 to Vaitses.
The Seeman patent above describes a method of constructing boat hulls using fiberglass, wherein a surface defining base material of a pliable sheeting, for example, a loosely woven fabric or elastic mesh reinforced with springy rods is attached to a skeleton framework which readily and easily assumes and forms the contoured curved surface of the boat hull, with subsequent addition of a resin and hardening of the resin. The disadvantages of this type of boat hull construction are many. Foremost, however, are the disadvantage of the necessary employment of a skeleton framework and the complicated multi-step or stage procedure necessary to apply the various layers which eventually make up the contoured curved structure of the boat hull. Accordingly, a method such as described in the Seemann patent is not practical from the standpoint of providing a low-cost yet strong fiberglass boat hull.
The Vaitses patent describes an improved method of forming a laminated boat hull using a mold, thereby eliminating the skeleton structure employed in the Seemann patent. The method described in Vaitses comprises the steps of laying up on the mold a first skin, e.g., a settable resin impregnated fiberglass fabric; placing upon the first skin a first set of substantially parallel spaced elongated members, e.g., polyurethane foam members; laying a continuous membrane adhering to the exposed portions of the first skin; placing a second set of substantially parallel spaced elongated members upon the continuous membrane so as to substantially fill the interstitial valleys between the members of the first set; and laying up a second skin, e.g., resin impregnated fiberglass fabric upon the second set of elongated members, and upon the exposed portions of the continuous membrane so as to complete the boat hull.
The method of fabricating in the Vaitses patent suffers from various disadvantages, the major disadvantage of which is the necessity of placing first and second sets of spaced members in a particular relationship to one another and in a particular spaced relationship of the members within each set. This requires time-consuming placement of individual members with the great possibility that structural strength in the boat hull can be lost due to misplacement or misalignment of elements. This, coupled with the fact that the method of construction described in the Vaitses patent requires a great number of steps to be carried out in a particular sequence and time interval makes such method somewhat less than totally satisfactory.
Copending application Ser. No. 508,690 filed Sept. 23, 1974, now U.S. Pat. No. 3,920,871, in the name of Frederick M. Johnson and directed to Woven Structural Element, Method of Manufacture Thereof, and Method of Making a Boat Hull Therefrom, such application and this application being commonly assigned, discloses an improvement over the foregoing constructions, utilizing a plastic foam in the fabrication of a laminated boat hull, providing a product with a high strength to weight ratio. The structural element which is described in such copending application broadly comprises parallel rows of alternately oppositely undulated bundles of substantially untwisted fiberglass rovings and a series of parallel foamed plastic slats extending transversely of such rows and alternately under and over adjacent bundles so as to define a woven structural element of foamed plastic slats and bundles of fiberglass rovings. The weave of the woven structural element allows the element to take the form of a curved surface, preferably the contoured curved surface of a boat hull. Impregnation of the structural element with a settable resin and subsequent setting of the resin produces a structural element having a high strength to weight ratio. The resin impregnation of the woven structure provides for I-beams of resin between the foamed plastic slats with X-reinforcement of glass rovings therein. In utilization of such structural element in the production of a boat hull, the resin impregnated structural element is utilized as a central buoyancy element between first and second face sheets of resin impregnated fiberglass matting or woven fiberglass cloth.
While the structural element described in such copending application is extremely effective as the central buoyancy element in the fabrication of a laminated boat hull, the weaving of fiberglass rovings and foamed plastic slats increases the cost of the fiberglass boat hull. Accordingly, the art has still sought an inexpensive stiffening element for the fabrication of boat hulls, which can create the desired buoyancy and stiffening at extremely low cost.
It has previously been known that cuts or slits can be formed in a foam product to achieve various results. For example, a product manufactured by Owens-Corning under the name GLASSPAK, in accordance with U.S. Pat. Nos. 3,627,603 and 3,647,588 to J. W. Greig has a plurality of score lines, some of which start at one surface of the foam product, and others of which start at the opposite surface. Since this is a unitary foam product, however, these score lines or slits do not provide the necessary flexibility for utilization of such foam product in the fabrication of a curved surface, such as the contoured curved surface of a boat hull. Accordingly, prior to the present invention, the art still sought an inexpensive stiffening element providing stiffness and buoyancy for the fabrication of a boat hull, which stiffening element was extendable in a plurality of directions, making the same eminently useful in the fabrication of the contoured curved surface of a boat hull.